Microbicides are products that are designed to be applied to vaginal or rectal mucosa with the intent of preventing or at least significantly reducing the acquisition of sexually transmitted infections including Human Immunodeficiency Virus (HIV). There is increasing epidemiologic evidence that rectal intercourse has the highest risk of HIV acquisition, perhaps 10 to 20 times more risk than unprotected vaginal intercourse, and that this risky behavior is increasing among women and men. Given the prevalence of this high-risk behavior it is important to develop microbicide gels optimized for rectal administration.